Well, we didn’t had any of those when it was decided that alcohol was going to be legal but weed wasn’t. Since then the amount of car accidents caused by alcohol was so high that someone invented ways to detect that. Had weed been legal we probably would already have similar detectors.

But also, those tests are not that accurate, if you use a mouthwash with alcohol you can blow up positive for example, and if you get into the blood tests I’m fairly confident THC can be detected in blood tests. But at the end of the day we shouldn’t be trying to measure that, we should try to measure the impact, e.g. have a test where the person needs to press a button when a light goes on to measure response time, or similar tests. That’s what you care about at the end, someone might be wasted with a single beer and not have proper reaction time anymore, while others might be fine even after a few. The problem with that is that everyone thinks they’re the latter, but they aren’t.